1. Field of the Invention
The invention relates to a sewing-material workpiece holder for an automatic sewing machine. It also relates to a method of operation by which the sewing-material workpiece holder closes.
2. Description of Background Art
Sewing-material holders have been known in general for a long time. One example is disclosed in German Patent 3,546,238, the disclosures of which are incorporated by reference, as are the other prior art materials cited herein. In these devices, the workpieces to be sewn to each other are placed in the sewing-material holder, fixed in position by clamping, and then sewn together.
The known sewing-material holders customarily consist of a thin bottom plate and a corresponding upper plate which can be swung shut in the manner of a lid. After closing the sewing-material holder, the pieces of material which are to be sewn to each other are held clamped between the two plates. Both the upper plate and the lower plate have recesses which correspond to the seam to be produced.
A disadvantage of these holders is that the previously carefully aligned individual parts can be relatively displaced when this clamping occurs, in particular when handling larger pieces of material, with correspondingly large-dimensioned sewing-material holders. The reason for this relative displacement is that when the upper plate swings down onto the lower plate, the pieces of material to be sewn are not immediately clamped over a large portion of their area, but rather the area clamped starts from a small, linear contact area at the rear of the sewing-material holder, and is increasingly enlarged until the sewing-material holder is completely closed.
As a result of this shifting of the workpieces, it is not possible to sew them together exactly in accordance with their original alignment. Therefore, these sewing-material holders cannot be used when it is particularly important for the subsequent formation of the seam to take place at a previously precisely defined place, for instance when sewing air bags.
The same problem occurs when a workpiece is to be sewn to a very flexible material, such as foam. Because of the great flexibility of the foam material, the latter is squeezed and pushed together to form a bulge when the workpieces are clamped.
It is possible to conceive of a sewing-material holder in which the upper plate is not swung about any axis or pivot onto the lower plate, but rather is lowered onto it vertically in a straight line. Such a holder would, however, have the disadvantage that the lower plate would no longer be properly visible to the operator, and would no longer be easily accessible because of the guide rods that would be required to guide the upper plate vertically. Because of these problems, with such a hypothetical sewing-material holder, it would be expected to be excessively difficult to place and align the pieces of material to be sewn.